Copper-based brazing alloys are known, for example, from EP 0 103 805 A2. The copper-based brazing alloys described in that document have a structure that is at least 50% amorphous and a composition which consists of 5 to 52 atom % of nickel, 2 to 10 atom % of tin and 10 to 15 atom % of phosphorus, remainder copper and incidental impurities. The total amount of copper, nickel and tin is in this case in the range from approximately 85 to 90 atom %.
Furthermore, RU 2041783 C1 has disclosed an amorphous copper-based brazing alloy which consists of 5 to 20 atom % of nickel, 20 to 10 atom % of tin, 10 to 15 atom % of phosphorus, remainder copper to which one or more of the elements gallium, indium, bismuth, lead, cadmium and/or zinc is added in quantities from 0.01 to at most 0.5 atom % to improve the wetting properties.
Both the copper-based brazing solders described above include phosphorus as an alloying element, since this element can lower the melting point and therefore the working point of the brazing solder, compared to other copper-based brazing solders. Moreover, the brazing solders described above have inherent flow properties, on account of their phosphorus content, and can be used for the cohesive joining of copper and copper alloys, for example brass, without the need for any flux. The copper-nickel-tin-phosphorus brazing solders described above have liquidus points of well below 750° C. and therefore represent the copper-based brazing solders with the lowest working points of all.
The copper-nickel-tin-phosphorus brazing alloys described above can be produced as powders, pastes, wires or amorphous foils. Powders are typically produced by melt atomization. Pastes are produced by mixing the metal powders with organic binders and solvents.
However, the intrinsic brittleness of the copper-nickel-tin-phosphorus alloys described means that the rapid solidification technique is the only way of producing brazing solders of this type in the form of homogenous and ductile foils.
It has been found that the copper-nickel-tin-phosphorus alloys described above have a tendency to be oxidized very extensively at the surface, in particular if they are exposed to a high level of atmospheric humidity for a prolonged period of time, so that discoloration and spots are formed on the surfaces of the alloy strips produced. The foil surfaces then have violet and/or greenish and/or bluish discolorations, which may extend over large parts of the foil. This phenomenon cannot be satisfactorily remedied even by the teaching of RU 2041783 C1. The additions of gallium, indium, cadmium and zinc disclosed in that document provide very little, if any, protection against surface oxidation.
The extensive surface oxidation which occurs may have a very adverse effect on the soldering properties of the alloys described. In particular, the flow and wetting properties deteriorate markedly.
Furthermore, the joining locations may be only incompletely filled with brazing solder, and consequently the mechanical stability of the parts to be joined can no longer be reliably ensured. Joining defects of this nature when brazing heat exchangers or other similar products can then lead to a considerable drop in the heat transfer rates required of them.
Hitherto, this problem has been combated by expensive packaging, in particular under vacuum, of the copper-based brazing alloys described in the introduction, in order to prevent surface oxidation even after prolonged storage in hot and/or humid regions.
However, this complex packaging incurs considerable additional costs during production, packaging itself and storage.